Radiotherapy techniques have improved dramatically in recent years. One particular improvement is the integration of radiotherapy with image acquisition devices such as cone-beam computed tomography (CBCT) scanners. In these systems, a moveable (e.g. rotatable) gantry supports a source of therapeutic radiation (i.e. in the megavolt range) as well as a source of imaging radiation (i.e. in the kilovolt range). A detector is positioned on the gantry substantially opposite the source of imaging radiation, and acquires imaging data as the gantry rotates around the patient. Such an imaging device allows efficient data collection for off-line decision protocols, validation of the patient geometry during actual therapy and detection of intra-fraction motion in two dimensions as well as cone-beam computed tomography (CBCT) to verify the patient anatomy in three dimensions.
Unfortunately, lateral scatter of the megavoltage (MV) beam into the kilovolt (kV) detector has a considerable impact on image quality. In general, the MV scatter depends on patient geometry, dose rate and field shape, and therefore it has been a prerequisite of conventional techniques to include values for these parameters in the CBCT reconstruction.